Due to the increasing energy demand, offshore oil and gas production is moving into deeper waters. For ensuring an efficient and secure production, processing facilities are being installed at the ocean floor. Recent subsea installations include an increasing amount of electrical equipment, including electrical components installed at the wellhead and Christmas tree, such as actuators and sensors. Furthermore, electrically driven compressors and/or pumps may be provided at a subsea installation.
For operating such electrical equipment, communication and control is required, as well as the supply of electric power. Communication and the supply with electric power may occur via an umbilical from a topside installation, or via subsea cable. A modem diplexer unit (MDU) may for example be provided for communicating with a topside installation, and well control units (WCU) or other subsea control units may be provided, e.g. for controlling equipment at a well head or the like.
When several such subsea electrical devices or units are installed, they need to be electrically connected for receiving power and for communication purposes. Cables, such as subsea jumper cables and oil filled hoses in which conductors are arranged are used for this purpose. Such electric connections generally constitute a weak link. Water ingress into such subsea cable or hose may cause a failure of insulation, which in turn can lead to a short circuit between the conductors provided in the cable or the hose, or may result in the flow of current from an electric power carrying cable or conductor to ground (earth or ground fault). Also, a fault may occur inside subsea electric equipment. The occurrence of such failure or fault can jeopardize the operation of the whole subsea installation.
A particular difficulty lies in finding the location of the failure of fault, and in isolating the failure or fault so that operation of the remaining subsea installation can continue. Trying to detect and find the location of a failure or fault from a topside installation is generally difficult, since galvanic isolation may be provided between topside and subsea electric systems, for example due to the presence of a transformer or the like. One possibility is the use of line insulation monitors (LIMs) for detecting a degradation of the insulation resistance. If more than one LIM is provided, the LIM measurements interfere with each other, resulting in inaccurate current readings. Furthermore, the LIMs consume electric power, resulting in an increased power demand and an increase in heat production, the waste heat being difficult to dispose of in subsea control units.